Jamieson, Fausset and Brown on Isaiah 12:3

3. draw water ... salvation--an expressive image in a hot country. On the last day of the feast of tabernacles the Jews used to bring water in a golden pitcher from the fountain of Siloam, and pour it, mingled with wine, on the sacrifice on the altar, with great rejoicing. This is the allusion in Jesus' words on "the last day of the feast" (Joh 7:2, 37-39). The pouring out of water indicated repentance (1Sa 7:6; compare, as to the Jews' repentance hereafter, Zec 12:10). There shall be a latter outpouring of the Spirit like the former one on pentecost (Joe 2:23).

wells--not mere streams, which may run dry, but ever-flowing fountains (Joh 4:14; 7:38), "Out of his belly (that is, in and from himself)--living water" (Is 42:18; Psa 84:6; Zec 13:1; Re 7:17).